1. Field of the Invention
The present invention relates to Micro Electro-Mechanical Systems (MEMS), and more particularly to the magnetic actuation of a MEMS micro-relay using latchable magnetic materials.
2. Description of the Related Art
Magnetic forces can be used to induce mechanical motion ("actuation") of magnetic materials. Electric current flowing through a conductor (e.g. a wire) induces a magnetic field around the conductor due to Faraday's law of induction (e.g. an electromagnet), and this mechanism is used to induce mechanical motion in many applications. Some implementation examples of electromagnetic actuation includes mechanical relays, bells used for fire alarm systems, and magnetic levitation trains.
Mechanical relays generally consist of a mobile mechanical electrode that is pulled into contact with a stationary electrode via magnetic force. In the most general implementation, a magnetic material is attached to the mobile electrode, and an electromagnet is positioned opposite the magnetic material on a stationary electrode or other stationary surface. Actuation of the electromagnet creates a magnetic field gradient that reacts with the magnetic field of the magnetic material attached to the mobile electrode and thereby causes the mobile electrode to be pulled or pushed (i.e. attracted or repelled) toward or away from the stationary electrode in a normally-open or normally-closed switch state of the relay, respectively.
Similar magnetic switching mechanisms have been used to actuate MEMS relays. In these applications, current flowing through an actuation coil pulls the mobile micromachined electrode toward a stationary electrode. Although such an actuation mechanism can deliver large actuation forces, the current required to maintain the switch in the on-state requires undesirable dissipation of a large amount of power (e.g. hundreds of milliwatts) in the control circuit of the relay. Such high power dissipation limits the integration of MEMS relays into CMOS circuitry where the amount of power thus dissipated results in an operation bottleneck, and also prevents high-density integration of such relays.
The operation of relays with low power dissipation becomes important as the relay density increases. In MEMS-based relays, in particular, power dissipation is an important issue since the power handling capability of the substrates is limited. Prior art thermal actuators dissipate too much power (typically a few hundred milliwatts) because the induced temperature change must be maintained in order to secure the switch state. Similarly, prior art magnetic actuators utilizing magnetic fields from a current source also dissipate a large amount of power (typically a few hundred milliwatts) in that the applied magnetic field must be maintained in order to secure the switch state. In applications where non-volatile switching is necessary, there is currently no solution to this power dissipation problem for MEMS micro-relay implementation.
Electrostatic actuators use an applied voltage across a parallel-plate capacitor to induce an attractive force between the two plates and, as such, do not need to dissipate as much power as thermal and magnetic actuators to maintain a switched state, although the actuation voltage must be maintained. Since there is no current flow through a pair of capacitor plates, this actuation mechanism does not dissipate any power to maintain the actuation status (i.e. the switched status in a MEMS relay). There are, however, two drawbacks in this actuation scheme. The first is that although no power need be dissipated to maintain the switched status, the potential difference between the two capacitor plates must be maintained. Thus, a power failure causes the actuation status to be lost. The second drawback is that the force that can be provided by the electrostatic actuator is limited to a few micro Newtons, thereby limiting the application of such an actuator.